Polycrystalline diamond (PCD) material comprises a mass of inter-grown diamond grains and interstices between the diamond grains. PCD material may be made by subjecting an aggregated mass of diamond grains to a high pressure and temperature in the presence of a sintering aid such as cobalt, which may promote the inter-growth of diamond grains. The sintering aid may also be referred to as a catalyst material for diamond. Interstices within the sintered PCD material may be wholly or partially filled with residual catalyst material. PCD may be formed on a cobalt-cemented tungsten carbide substrate, which may provide a source of cobalt catalyst material for sintering the PCD.
PCD material may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. For example, tool inserts comprising PCD material attached to a substrate are widely used in drill bits for boring into the earth in the oil and gas drilling industry. In many of these applications, the temperature of the PCD material may become elevated as it engages, at high energy, rock or other workpieces or bodies.
PCD is extremely hard and abrasion resistant, which is the reason it is the preferred tool material in some of the most extreme machining and drilling conditions, and where high productivity is required. Unfortunately, PCD suffers from a number of disadvantages, several of which are associated with the metallic binder material typically used. For example, metal binder may corrode in certain applications such as the high speed machining of wood. In addition, metals or metal alloys are relatively soft and susceptible to abrasion, reducing the average wear resistance of the PCD material. However, the most problematic aspect of PCD is arguably its relatively poor thermal stability above about 400° C.
In use, the temperature of a PCD working element at a working surface may approach 1,000° C. in certain applications such as rotary rock drilling. Heat tends to degrade PCD in two principal ways, namely by inducing thermal stress arising from differences in thermal expansion of the diamond, the binder and the substrate, and by inducing the diamond to convert to graphite, which is the thermodynamically stable phase of carbon at ambient conditions. The former mechanism sets in above about 400° C. and becomes progressively more significant as the temperature is increased. The temperature at which the latter mechanism becomes significant depends on the nature, quantity and spatial distribution of the binder material in relation to the diamond. The most commonly used binder metals are selected because they catalyse the sintering of diamond at ultra-high pressures. Unfortunately, these same metals also catalyse the reverse process of diamond conversion to graphite (or “graphitisation”) at lower pressures. In the most typical case where the binder is Co, significant graphitisation is believed occur above about 750° C. in air.
The working life of tool inserts may therefore be limited by graphitisation of the superhard material at high working temperatures which could induce spalling and chipping.
Polycrystalline diamond compacts (PDC) which may form cutting tools for use in drill bits in industrial applications such as drilling in the oil and gas industry, are therefore often exposed to extremes of pressure and temperature in hostile, abrasive and erosive environments.
Operating temperatures experienced by a cutting element whilst drilling are thought to have a major effect on the tool life and general durability of these PDC cutters. However, there is still much uncertainty and debate around what the range of actual cutting temperatures might be as conventional sensors for temperature and pressure are unable to survive during the drilling process.
Information relating to the environment being drilled and the performance of the cutter would be useful for drill bit operators as it may enable the characterization and evaluation of the durability, performance, and potential failure of the drill bit.
There is therefore a need to provide a method and apparatus for obtaining information relating to performance and/or behaviour of a drill bit and related components whilst the drill bit is in use.